Transport across the nuclear membrane is required at numerous steps in the life cycle of the human immunodeficiency virus (HIV) and other retroviruses. one in the cytoplasm, HIV RNA is reverse-transcribed into double-stranded DNA which then must enter the nucleus. This is the preintegration complex which contains DNA, RNA, reverse transcriptase integrase and matrix protein. A growing amount of evidence points to the viral matrix protein of HIV as a mediator of the nuclear transport event. Efforts are underway to determine how the matrix protein may be involved in nuclear transport of the DNA-RNA-protein complex. In recent studies, the matrix protein has been isolated in recombinant form and its nuclear localization examined. Other retroviral regulatory proteins, also enter the nucleus to perform their function, and viral transcripts are exported to the cytoplasm. The viral proteins, rev and tat, have been identified as key regulators of the transcription and transport of HIV envelope mRNA out of the nucleus. These proteins contain sequences that are necessary and sufficient for targeting to the nucleolus. An in vitro system using permeabilized cells has been developed to look at the nucleolar targeting of proteins bearing such targeting signals. We have observed that transport to the nucleolus is an active process. Recently, it has been demonstrated that a reduction in cellular GTP levels induced by inhibitors of IMP-dehydrogenase is correlated with a reduction in the nucleolar accumulation of other proteins. We have found that these inhibitors also block the nucleolar targeting of HIV rev protein. However, these inhibitors, which include mycophenolic acid, did not affect nucleolar accumulation of the model protein in permeabilized cells. Moreover, 100-fold reduction of GTP levels in the in vitro assay did not affect nucleolar accumulation of the model protein. Efforts are underway to identify cytosolic components required for nucleolar transport.